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Water in polyurethane networks: physical and chemical ageing effects and mechanical parameters
Continuum Mechanics and Thermodynamics ( IF 2.6 ) Pub Date : 2022-03-10 , DOI: 10.1007/s00161-022-01082-y
Wulff Possart , Bettina Zimmer

The chemical structure, polymer mobility and mechanical properties are studied for a cross-linked amorphous poly(ether urethane) (PU) from glass transition to rubber elasticity for juvenile dry samples and for water-saturated states after exposure to humid air (r.h. = 29, 67, 95, 100%) at \(60~^\circ \hbox {C}\) during 1 y of ageing. For saturated samples, network chain cleavage is the chemical ageing mechanism, but it is too weak and slow to affect on the physical properties significantly within 1 y. Water acts primarily in a physical manner. Within 1 d, \(\hbox {H}_{{2}}\hbox {O}\) molecules replace part of the weak urethane H-bonds by \(\hbox {H}_{{2}}\hbox {O}\)–urethane H-bonds and reduce all other physical interactions between network chains by solvating hydrophilic segments. Thus, the cooperative polymer mobility strongly amplifies: The gain of specific conformational entropy doubles across the caloric glass transition, which shifts by −17 K. A \(\hbox {H}_{{2}}\hbox {O}\) concentration of only \(\hbox {c}_{{\mathrm{H}_2\mathrm{O}}}~\approx ~(0.4~\ldots ~0.5)~\hbox {c}_{\mathrm{\mathrm{H}_2\mathrm{O},max}}\) suffices for the major part of these fast rearrangements. Some part of the water slowly forms (during 3–4 months) a finely dispersed water-rich mixed phase with the PU chains. Except the new phase, these molecular processes of physical ageing strongly affect the mechanical properties at damage-free deformation. For dry PU in the glass transition, the shear modulus, \(\mu _{\mathrm{relaxed}}\)(T), after viscoelastic stress relaxation only depends on the deformation-induced entropy change—like in the rubber elastic state. Within one month, water drastically decreases the viscoelastic response, as expected for plasticisation. However, \(\mu _{\mathrm{relaxed}}\)(T) slightly grows in wet PU. \(\hbox {H}_{{2}}\hbox {O}\) molecules cause these opposite trends by boosting the cooperative mobility (i.e. extension of the accessible conformational space and entropy by reduction in energy barriers) and by occupation of free volume compartments. Water quickly reduces the fracture parameters by about 50%. We explain that embrittlement by the \(\hbox {H}_{{2}}\hbox {O}\)-induced facilitation of cooperative network chain motions, which let fracture proceed with less energy. In summary, our findings provide a detailed conception of the molecular effects the \(\hbox {H}_{2}\hbox {O}\) molecules have on the PU network, and they explain the consequences for the mechanical properties.



中文翻译:

聚氨酯网络中的水:物理和化学老化效应和机械参数

研究了交联无定形聚醚聚氨酯 (PU) 从玻璃化转变到橡胶弹性的化学结构、聚合物流动性和机械性能,用于幼年干燥样品和暴露于潮湿空气后的水饱和状态 (rh = 29 , 67, 95, 100%)在 1 年老化期间在\(60~^\circ \hbox {C}\) 。对于饱和样品,网络链断裂是化学老化机制,但它太弱且太慢,在1 y内无法显着影响物理性质。水主要以物理方式起作用。在 1 d 内,\(\hbox {H}_{{2}}\hbox {O}\)分子用 \ ( \hbox {H}_{{2}}\hbox替换了部分弱氨基甲酸酯氢键{O}\)-氨基甲酸酯氢键并通过溶剂化亲水链段减少网络链之间的所有其他物理相互作用。因此,协同聚合物迁移率强烈放大:特定构象熵的增益在热量玻璃化转变过程中翻倍,其移动了 -17 K。 A \(\hbox {H}_{{2}}\hbox {O}\)仅集中\(\hbox {c}_{{\mathrm{H}_2\mathrm{O}}}~\约 ~(0.4~\ldots ~0.5)~\hbox {c}_{\mathrm{\ mathrm{H}_2\mathrm{O},max}}\)足以满足这些快速重排的主要部分。部分水与 PU 链缓慢形成(在 3-4 个月内)精细分散的富水混合相。除了新阶段,这些物理老化的分子过程强烈影响无损伤变形时的力学性能。对于玻璃化转变中的干 PU,粘弹性应力松弛后的剪切模量\(\mu _{\mathrm{relaxed}}\) (T) 仅取决于变形引起的熵变化——就像在橡胶弹性状态下一样. 在一个月内,水会显着降低粘弹性响应,正如塑化所预期的那样。然而,\(\mu _{\mathrm{relaxed}}\) (T)在湿 PU 中略微增长。\(\hbox {H}_{{2}}\hbox {O}\)分子通过提高协同迁移率(即通过减少能垒来扩展可接近的构象空间和熵)和通过占据自由体积隔间来引起这些相反的趋势。水会迅速将断裂参数降低约 50%。我们解释了由\(\hbox {H}_{{2}}\hbox {O}\)引起的脆化 -诱导协同网络链运动的促进,这使得断裂以更少的能量进行。总之,我们的研究结果提供了\(\hbox {H}_{2}\hbox {O}\)分子对 PU 网络的分子效应的详细概念,并解释了对力学性能的影响。

更新日期:2022-03-10
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